In recent years, the 3GPP (3rd Generation Partnership Project) and the 3GPP2 (3rd Generation Partnership Project 2), which are standards organizations for third-generation mobile phone standard, have been pursuing the development of specifications of a mobile communication system capable of the co-existence of a plurality of communication networks with different communication methods. As such a mobile communication system, for example, there is known a mobile communication system in which a 1×+HRPD (High Rate Packet Data) network for performing voice communication and packet communication co-exists with an LTE (Long Term Evolution) network for performing packet communication only. In the mobile communication system in which the 1×+HRPD network co-exists with the LTE network, a wireless communication device such as a mobile phone (hereinafter, referred to as “UE (User Equipment)” is assumed to perform handover from the 1×+HRPD network to the LTE network or handover from the LTE network to the 1×+HRPD network.
When performing handover from the 1×+HRPD network to the LTE network, the UE performs location registration to the LTE network beforehand. Namely, the UE transmits an Attach Request which is a signal requesting location registration to the LTE network. Then, the UE, which has transmitted the Attach Request, receives a response signal indicating a response to the Attach Request from the LTE network. When the UE has received an Attach Accept indicating acceptance of the Attach Request as the response signal from the LTE network, the UE sends a reply of an Attach Complete notifying of completion of the receipt of the Attach Accept to the LTE network. With this, the location registration to the LTE network is completed, and after that, a wireless link between the UE and the LTE network is established, so the UE can execute packet communication with the LTE network without performing a location registration procedure.    Japanese Laid-open Patent Publication No. 2010-68233
However, the above-described conventional technology has a problem that when the UE has had a voice communication request during execution of handover to a communication network for performing packet communication only, the execution of voice communication may be obstructed.
Specifically, in the above-described conventional technology, even when the UE has acquired a voice communication request to the 1×+HRPD network before receiving the Attach Accept, upon receipt of the Attach Accept, the UE sends a reply of an Attach Complete to the LTE network. Therefore, in the above-described conventional technology, a wireless link between the UE and the LTE network is established, and voice communication between the UE and the 1×+HRPD network may be obstructed.